1. Field of the Invention
The present invention relates to an oil recovery method and system for refrigeration apparatus using a screw compressor and, more particularly, to such an oil recovery method and system for low capacity operation of the compressor.
2. Discussion of the Related Art
Oil lubricated screw compressors are commonly used in refrigeration apparatus provided with an oil/refrigerant separator from which oil is fed back to the compressor whereas compressed refrigerant is passed from the separator, through the condenser, through the evaporator units of the system, and back to the suction inlet of the compressor. In certain applications, such as in refrigeration apparatus used for chilling water and other liquids, for example, efficient and compact packaging of the compressor, condenser, evaporator and separator components results in the suction inlet of the compressor opening downwardly to the top of the evaporator chamber. Because the working screws of the compressor are lubricated and in some measure sealed by oil, this geometry of refrigeration components presents a potential for oil dropping from the compressor through the suction opening thereof to the evaporator chamber.
During normal operation of refrigeration apparatus of the type mentioned, the compressor is operated at adequate gas flow through the compressor suction chamber to retain droplets of oil which are present. Under such conditions, the oil separator and recovery system provides adequate management of the oil in the apparatus. At lower compressor capacities, however, the velocity of gases entering the suction chamber of the compressor is reduced to a point where oil from the compressor may drop into the evaporator chamber. If such low capacity operation occurs for any substantial period of time, the oil accumulates in the evaporator and results in reduced efficiency of the refrigeration cycle performed by the apparatus. Also, the supply of oil needed for compressor lubrication may become inadequate.